The present invention relates to a non-woven fabric for use in a filter and a method for producing the non-woven fabric. In particular, the present invention relates to a non-woven fabric having an excellent filtering efficiency and a long lifetime, and a method for producing such non-woven fabric.
Recently, a non-woven fabric has come to be used as the filtering medium for an air filter, a filter for liquid material, etc. and the production of the non-woven fabric is increasing nowadays.
The non-woven fabric for use in a filter is preferred to have a large pore size at the inlet side (upstream side) and a small pore size at the outlet side (downstream side). To meet this requirement, it has been practiced to laminate a non-woven fabric having a larger pore size onto another non-woven fabric having a smaller pore size. However, in such a laminated non-woven fabric, since the pore size becomes discontinuously and abruptly decreased at the interface between the laminations, a solid matter to be filtered out tends to accumulate at the interface to lead to a pressure loss, thereby shortening the lifetime of a filter.
Alternatively, is proposed a method for controlling the pore size in which a sheet of non-woven fabric is subjected to calendering by being passed between a pair of rolls consisting of a metal roll and a metal roll coated with cotton, etc. (cotton roll) to produce a non-woven fabric having a relatively small pore size (greater packing density) on the side treated with the metal roll and a relatively large pore size (lesser packing density) on the side treated with the cotton roll. However, in a non-woven fabric so produced, although the packing density abruptly decreases from the surface treated with the metal roll towards the center of the non-woven fabric, a further decreasing in the packing density is little observed from the center to the surface treated with the cotton roll. In other words, the changing rate of the packing density is discontinuously and steeply changed at the vicinity of the center of the non-woven fabric. Therefore, a solid matter tends to accumulate at the vicinity of center, which is likely to cause a pressure loss to shorten the lifetime of the filtering medium.